Ionic conductivity is commonly associated with the flow of ions through a liquid solution of salts. In the vast majority of practical uses of ionic conductors, i.e., as electrolytes for dry cell and sealed lead acid batteries, the liquid solution is immobilized in the form of a paste or gelled matrix or is absorbed in a separator to overcome the difficulties associated with handling and packaging a liquid. However, even after immobilization, the system is still subject to possible leakage, has a limited shelf life due to drying out or crystallization of the salts and is suitable for use only within a limited temperature range corresponding to the liquid range of the electrolyte. In addition, the use of a large volume of immobilizing material has hindered the aims of miniaturization and lowers the output capacity.
Improved microelectronic circuit designs have generally decreased the current requirements for each transistor which makes up the electronic devices. This in turn has enhanced the applicability of solid electrolyte power sources which usually can deliver currents only in the microampere range. These solid electrolyte systems have the inherent advantages of being free of electrolyte leakage, corrosion and internal gassing problems due to the absence of a liquid phase. In addition, they also have a much longer shelf life than the conventional liquid electrolyte power sources.
In attempting to avoid the shortcomings of liquid systems, investigators have surveyed a large number of solid compounds seeking to find compounds which are solid at room temperature and have specific conductances approaching those exhibited by the commonly used liquid systems. Solid electrolytes must be essentially electronic insulators so as not to internally short the cell while at the same time they must allow for ionic migration if the cell is to operate properly. There are many solid state electrolytes "disclosed" in the art that can be used for solid state cells but many can only operate efficiently at higher temperatures, have low operating voltages or have internal high resistance.
United Kingdom Patent No. 2,201,287B discloses a solid polymer electrolyte for us in solid electrolyte cells which comprises a complex of a solid polymer and an alkali metal salt, which polymer is capable of forming donor-acceptor type bonds with alkali metal ions and is capable of conducting alkali metal ions and wherein the complex is associated with a mixture of more than one substituted or unsubstituted 1,3-dioxolane-2-ones. The preferred mixture recited is ethylene carbonate and propylene carbonate. This solid electrolyte has been found to produce a good lithium solid state cell that can operate at ambient temperature.
U.S. Pat. No. 5,089,027 discloses a method for producing a solid electrolyte cell using the solid electrolyte disclosed in the U.K Patent No. 2,201,287B referred to above. In particular, an adhesive coated frame is deposited on the peripheral area of current collector sheets and the components of the cell are positioned within the frame of adhesive whereupon the current collector sheets are then secured together at the peripheral area containing the adhesive layer.
Several cell applications require that the cell be directly incorporated into a device to produce a portable finished package. This could require the cell to be encapsulated or molded into the device. In injection molding, for example, the cell must be highly planar in appearance and capable of withstanding high temperature processing up to 200.degree. C. These conditions tend to favor the use of a solid electrolyte cell. The polymeric cell components such as the one referred to above, function well at elevated temperatures.
Flat solid electrolyte cells have been assembled typically with an adhesive coated substrate as a spacer and seal. The adhesive generally used has melting points in the range of 80.degree. C. to 105.degree. C. and unfortunately, the seal integrity of the cells is subject to failure at high temperatures.
It is an object of the present invention to provide a method for assembling a solid electrolyte cell within a ceramic frame in which the ceramic frame functions as a housing for the cell components and said frame is soldered to a conductive terminal sheet on each of its top and bottom surfaces.
It is another object of the present invention to provide a method for assembling a solid electrolyte cell within a ceramic frame, said cell employing a solid electrolyte film containing poly(ethylene oxide) or a poly(ethylene oxide) type polymer in conjunction with ethylene carbonate and propylene carbonate.
It is another object of the present invention to provide a solder sealed solid electrolyte cell.
The foregoing and additional objects will become more fully apparent from the following description and drawings.